Effects of δ Phase and Annealing Twins on Mechanical Properties and Impact Toughness of L-PBF Inconel 718

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The scale in Fig.13 and 14 is different, which is confused for comparison.

2. It would be better if a quantitative relationship between the δ phase ( or annealing twins) and mechanical properties could be established.

3. The abstract should be re-written.

4. Conclusion needs to be refined.

Comments on the Quality of English Language

The grammar can be polished.

Author Response

We are very thankful to the reviewer for the valuable comments and feedback. The revised/added texts are highlighted in green in the manuscript.

1. The scale in Fig.13 and 14 is different, which is confused for comparison.

Ans1: Images with equal magnification are now presented in both figures.

 

2. It would be better if a quantitative relationship between the δ phase (or annealing twins) and mechanical properties could be established.

Ans2: It is possible to demonstrate a relationship between the δ phase (or annealing twins) and mechanical properties, as shown in the literature. However, quantitative data are necessary to correlate these factors. To establish a quantitative relationship between the delta phase (annealing twins), heat treatments must be conducted at various hold times to obtain different volumes/quantities of delta precipitates for analyzing their effects on mechanical properties. Similarly, heat treatments at different temperatures/hold times need to be carried out to establish a relationship between annealing twins and mechanical properties. However, the current study was not designed to observe results that would allow us to draw such a conclusion.

3. The abstract should be re-written.

Ans3: The abstract has been revised accordingly.

4. Conclusion needs to be refined.

Ans4: The Conclusion has been revised accordingly.

The grammar can be polished.

Ans: The entire manuscript has been revised for grammar.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. The S1, S2, S3 in Fig. 9 should be defined in the manuscript.

2. The scale of Figs. 9, 11 and 12 are missing, especially the X-axis

Author Response

We are very thankful to the reviewer for the valuable comments and feedback.

1. The S1, S2, S3 in Fig. 9 should be defined in the manuscript.

Ans1: We have described the notations in the manuscript. The following sentence has been added in sub-section 2.3, which is highlighted in blue.

The surfaces are referred to as S1 (the surface normal to the building direction), and S2 and S3 (the other two surfaces).

2. The scale of Figs. 9, 11 and 12 are missing, especially the X-axis.

Ans2: The scales along the X-axis in all the figures refer to the orientation of the specimens (X, Y, and Z), or the specimens. The X-axis in all figures does not refer to numerical data.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Review JMMP IN718

 

Thank you, authors, for the provided article. A lot of work has been done.

I think, some refining is needed. Especially in the sense of improving correctness, e.g. be sure to differentiate enough for the reader between your wishes, hypothesis, evidenced results and referenced results.

 

You are speaking about delta phase and twins, but grainsize is also discussed and gamma double prime influence is not excluded. Influence of twins has not been evidenced in this article, just their presence. And more is discussed based on references. You have used different ageing treatment, but did not check the gamma double prime phase.

Maybe the Title shall be corrected accordingly. Or add evidences or references, which will support the elimination of other influences. Then check the self-consistency of the whole article, accordingly to chosen direction.

Separating the discussion paragraphs from the Results section will be useful.

Any comments to porosity, or carbides?

 

Further comments follow:

 

Abstract:

Line 12: Instead of 2 heat treatment “methods”, rather “set-ups” or “set of parameters”

 

Introduction

The first paragraph is too short. It should be mentioned, that parts from IN718 alloy are manufactured by hot working, or investment casting too, followed by appropriate HT.

Line 31: “other applications…”, but where are “main/principal applications”?

Line 38: “IN718 is a FCC austenite solid supersaturated solution matrix…” as far as I know, this is not generally valid. Maybe for as-printed part? But that is after this sentence. Maybe, for the context, it shall by given similar sentence about microstructure of hot worked or cast parts, in 1^st paragraph?

Page 2 of 22

Second paragraph, Line 59-60 – As far as I know, the gamma prime particles are generally bigger than gamma double prime particles. Or give the context, where it is your way.

Page 3 of 22

Ref 20 – Mentions that lack of delta is also not good for high temperature properties. Why is your ST aiming for no delta phase?

Ref. 22 – Different gamma prime and gamma double prime is mentioned as a co-factor, not only twins.

Why do you use different ageing treatment for different ST ?

 

Results

Any information about the printing powder, diameter?

2.2 Heat treatment

Why do you use different ageing treatment for different ST ?

 

2.3 Tensile test

Why was the straining rate changed? Why this way? From 0,015 to 0,01, that is slowing down?

Any standard reference for the HV measurement was followed?

 

2.4 Characterisation of microstructure

Maybe, shall be divided in separate paragraphs. Readers would follow more easily.

E.g divide line 228 from 229, 231 from 232 “Phases…

And 238 from 239 “X-ray…”

 

3 Experimental results

Sometimes the mechanical properties are given first, but this way is also ok.

 

3.1 use “and” instead of “cand” in the title

The subtitles should also have numbering, IMHO.

Fig. 2 Picture a) will by probably more appropriate in same magnification as b)

Can you add a small space between b) and c) to differentiate the two Sts?

 

The delta precipitates

I think the EDS results shall be given as well, if you are using them to distinguish the Laves phase and delta phase

Line 269 – “larger”, not “lager”

 

Figure 3

Can you give the orientations of the depicted specimens?

Please add arrows to Laves phase particles in a) and c), and for GB in d)

Details will be useful to support the differences in delta precipitates, or numerical values.

Can you add any supporting evidence for the sentence on line 285 and 286?

Any comment to preferential orientation of delta precipitates?, Fig. 3b

 

Any comment to figure 4? Why is it given here? Is it from a broken tensile specimen?

 

Fig. 5 – Where is the Laves phase reported in Line 291 to 298.

Fig. 5 – Orientations of the specimens 5a and 5b?

 

Line 294 – “well known..” – any reference?

 

Line 299 to 307 – belongs rather to Discussion section, but can be here.

Line 308 – 314 – Maybe, will be better in the beginning of the 3.1 section, or at the beginning of the Delta phase section

 

The annealing twins section – will be probably more appropriate between the Microstructure section and Delta phase section.

 

Page 10 of 2022

Line 336 and 338 - Probably “solution” instead of “solidification”

Line 338-339 - Any evidence or reference to “However, the subgrain boundaries…”.

 

Identification of delta phase

Please add a space between the title and previous paragraph.

This part is also about identification of other phases, as gamma double prime

 

Phase identification with X-ray

Maybe, will be more useful before the Delta precipitates section, since this X-ray measurement characterises bigger area and shows, that there is no delta in SS1100 specimen.

The texture coefficient – Maybe, will be more illustrative from EBSD? Since EBSD was provided in Twins section.

Line 382 – I do not see the “Obtained numerical ?results?”

 

Line 399 to 409 – Is rather Discussion, discusses delta, grain size, gamma ‘’.

Line 419 – 428 – Is rather Discussion.

Line 460 – 471 – Is rather Discussion.

Fig. 11 – Will be more useful next or closer  to Fig 10.

Line 487 – 494 - Is rather Discussion.

 

3.5 Fracture analysis

Fig. 4 – Will be more useful in this section

Line 508 - Any evidence from the fracture surface about the IG character of the microcracks and presence of larger delta precipitate?

Fig. 13 - More detailed images are needed for documentation of the “area of cleavage”. Any role of carbides?

Line 522 – 531 - Is rather Discussion

Fig 14, Line 544 – Typing error “coalesces”, or any word missing?

Fig. 14 – More detailed images are needed for documentation of the “?facets?”. Any role of carbides?

 

4.1

Line 563 – 566 – This is evidence of IG character of secondary microcracks. As a consequence of the specimen orientation?

But on fracture surface, mostly ductile dimples were shown. Probably TG?

 

Line 570 – What is the base for “attribution of high ductility to presence of twins”? References? Evidences? In previous parts of the Results section you have discussed also grain size. How did you separate this effect? Not talking about the gamma double prime. I think this separation is insufficient.

Possible influence of other factors is mentioned in the Conclusions part as well as in Abstract. Please, make all parts more consistent to the other parts.

 

Conclusions

 

How do you conclude which factor is “mainly responsible”?

Grain size, subgrains…It seems contradictory to the Title of the article and the introductory paragraph of the 5.Conclusions section.

Last point – Is true, but some advice that no delta alloy is not the best choice is e.g. in Ref 20.

Author Response

We are very thankful to the reviewer for taking his/her time to read thoroughly and provide us with detailed and useful comments and feedback. We have reviewed the specific comments and made the necessary corrections. If you have any further suggestions or concerns, please let us know. The revised/added texts in the manuscript are highlighted in yellow.  

Thank you, authors, for the provided article. A lot of work has been done.

Response: Thank you for the appreciation as well.

I think, some refining is needed. Especially in the sense of improving correctness, e.g. be sure to differentiate enough for the reader between your wishes, hypothesis, evidenced results and referenced results.

Response: We have addressed the specific comments that are relevant to this general comment.

You are speaking about delta phase and twins but grainsize is also discussed and gamma double prime influence is not excluded. Influence of twins has not been evidenced in this article, just their presence. And more is discussed based on references. You have used different ageing treatment but did not check the gamma double prime phase.

Response: Although the focus of this paper is on the delta phase and twins, it is also necessary to consider other factors that may be affecting mechanical properties simultaneously. It could be difficult to separate the contributions of the delta phase from the rest based on the current heat treatment scheme.

… Influence of twins has not been evidenced in this article, just their presence.

Response: We have provided several references and thoroughly discussed the topic. Please refer to our response below for a repeated comment towards the end.

… You have used different ageing treatment, but did not check the gamma double prime phase.

Response: We have demonstrated the presence of gamma double prime using electron diffraction and images, as well as X-ray diffraction techniques. Discussions have also been included on their effects where necessary. Please refer to the contents in lines 351-359, 370-375, and lines 460-462 for further details.

Maybe the Title shall be corrected accordingly. Or add evidences or references, which will support the elimination of other influences. Then check the self-consistency of the whole article, accordingly to chosen direction.

Response: We believe that we have provided sufficient evidence from the literature to support the current result. Therefore, we may not need to change the title since it is essential to address factors that act simultaneously wherever necessary in the article.

Separating the discussion paragraphs from the Results section will be useful.

Response: In the 'Discussion' section, our main focus is on the delta phase and annealing twins to align with the title. Therefore, we have highlighted certain issues in the 'Results' section where further elaboration and supporting details are required. This method is commonly used by many scholars in journals.

Any comments to porosity, or carbides?

Response:

Porosity

Porosity analysis was not conducted in this study since the effects are expected to be the same for both sets of specimens. They were printed under the same process parameters simultaneously in the same chamber. In addition, the impact of porosity on these tests is minimal, as printing was carried out using the optimized energy density recommended by the AM machine manufacturer.

Carbides

The presence of carbides, especially NbC, can reduce the amount of Nb available for the precipitation of hardening precipitates. Through heat treatment, carbides can be fully or partially dissolved depending on the peak temperature and holding time. In the current study, which exclusively utilizes homogenized specimens, we did not observe the presence of carbides with EDS and SEM images. The undissolved carbides may be small and require careful examination for identification. Therefore, they could have some effects, but we did not consider the impact of carbides.  Similar response is given below.

Further comments follow:

 

Abstract:

Line 12: Instead of 2 heat treatment “methods”, rather “set-ups” or “set of parameters”

 Response: It is corrected since the abstract has revised.

Introduction

The first paragraph is too short. It should be mentioned, that parts from IN718 alloy are manufactured by hot working, or investment casting too, followed by appropriate HT.

Response: We do not find a revisiting of traditional manufacturing methods relevant here. We are entirely focused on AM methods, specifically on L-PBF-fabricated parts.

Line 31: “other applications…”, but where are “main/principal applications”?

Response: A sentence which was missed during editing is now added.

‘Because of this, IN718 is used in environments of high temperatures, such as aerospace, nuclear, chemical, and petrochemical industries where high strength, corrosion resistance, and fatigue life are prior requirements.’

Line 38: “IN718 is a FCC austenite solid supersaturated solution matrix…” as far as I know, this is not generally valid. Maybe for as-printed part? But that is after this sentence. Maybe, for the context, it shall by given similar sentence about microstructure of hot worked or cast parts, in 1^st paragraph?

Response: It is corrected and written as follows:

‘IN718 is a face-centered cubic, austenite (g) solid supersaturated solution matrix rich in Ni, Cr, and Fe, intermetallic laves phase (Ni, Fe, Cr)₂(Nb, Mo and Ti), and precipitated phases of IN-718 alloy that mainly consist of γ′′ (Ni3Nb) phase, γ′ (Ni3(Al, Ti)) phase, δ (Ni3Nb) phase, and MC carbides [References].’

Page 2 of 22

Second paragraph, Line 59-60 – As far as I know, the gamma prime particles are generally bigger than gamma double prime particles. Or give the context, where it is your way.

Response: Under standard aging conditions (e.g. 720/8h + 650/8h), gamma double prime is larger than gamma prime. This is supported by evidence in reports in the literature, including one of our previous papers (reference #7). Of course, it is possible to find conditions where gamma prime could be larger than gamma double prime. There are some case studies, such as the early works by Cozar and Pineau in Metall. TrResponse. V4, Jan 1973, that studied the coarsening behaviors of the two precipitates using non-standard alloys (varying the ratio of Ti+Al/Nb and longer hold time). In this study, it was observed that gamma prime coarsened under longer hold times at a single aging temperature, which is not within the temperature range that favours precipitation and coarsening of gamma double prime. The relevant references are listed below.

Tucho, W.M. and V. Hansen, Characterization of SLM-fabricated Inconel 718 after solid solution and precipitation hardening heat treatments. Journal of Materials Science, 2019. 54(1): p. 823-839.

1. Sundararaman, P. Mukhopadhyay, and S. Banerjee, Some Aspects of the Precipitation of Metastable Intermetallic Phases in Inconel 718, Metall. Trans. A 23A (JULY 1992).

 

Han, Y. fang, Deb, P., & Chaturvedi, M. C. (1982). Coarsening behaviour of γ″- and γ′-particles in Inconel alloy 718. Metal Science, 16(12), 555–562.

 

Page 3 of 22

Ref 20 – Mentions that lack of delta is also not good for high temperature properties. Why is your ST aiming for no delta phase?

Response: The effects of the delta phase on different properties of IN 718 are controversial in the literature. Most reports indicate poor performance in terms of creep resistance, ductility, impact toughness, etc. A few reports support the delta phase, citing beneficial effects, such as reference 20. Contrary to reference 20, a good report on high-temperature stress rupture life is the article by C.M. Cuo et al. (Mater Sci & Eng. A 510-511 (2009) 289-294). This report shows that the sample ST at 1095/1h + aged exhibited a creep life of 300 hours, which is twice that of samples with δ precipitates (1095/1h + 955/1-4h + aged) (150 hours). Similarly, the elongation to failure of the former condition is higher than the latter (5.3% vs. 1.4%). This indicates that careful and more systematic studies are needed to reconcile these conflicting reports. Our aim in avoiding the delta phase in one set of specimens was to study impact toughness and other properties to determine the contribution of the delta phase. What we have understood about the effects of the delta phase is that it depends on volume fraction, distribution, and in some instances on morphology. Generalizing as always having a positive or negative impact may be incorrect.

Ref. 22 – Different gamma prime and gamma double prime is mentioned as a co-factor, not only twins. Why do you use different ageing treatment for different ST?

Response: We greatly appreciate the reviewer's observation in this regard. It would have been nice if it were done like that, unfortunately, it was not. Our original experiment design included three sets of specimens: set A (1100/3h + 760/8h + 620/8h), set B (1100/3h + 700/12h), and set C (980/1h + 760/8h + 620/8h). Unfortunately, we were unable to investigate set A due to a shortage of specimens provided by the manufacturer/supplier. We plan to include this set in our future investigations to ensure a more systematic and comprehensive study.

 

Results

Any information about the printing powder, diameter?

Response: It is good to have such information, including details of process parameters, but unfortunately, we didn’t receive any data about the powder from the supplier.   

2.2 Heat treatment

Why do you use different ageing treatment for different ST?

Response: Please see the response to the previous comment. Our aim was to study samples that underwent similar aging treatments but different STs. However, a set of samples was missed due to a misunderstanding with the supplier. We tried to obtain them, but it did not materialize for a long time during the study period. 

2.3 Tensile test

Why was the straining rate changed? Why this way? From 0,015 to 0,01, that is slowing down? Any standard reference for the HV measurement was followed?

Response: The standards followed for the measurements of both tensile strength and hardness are now added as references #28 and #29. However, there is a mistake in the second figure stated for the strain rate. It should be 0.02 instead of 0.01. This correction has been made in the manuscript as well. Once the yield strength value is determined, increasing the strain rate may not affect the measurement, rather it speeds up the testing time.

2.4 Characterisation of microstructure

Maybe, shall be divided in separate paragraphs. Readers would follow more easily.

E.g divide line 228 from 229, 231 from 232 “Phases…

And 238 from 239 “X-ray…”

 Response: The comment is accepted and corrected accordingly.

3 Experimental results

Sometimes the mechanical properties are given first, but this way is also ok.

 Response: agree.

3.1 use “and” instead of “cand” in the title

Response: corrected.

The subtitles should also have numbering, IMHO.

Response: It is done now.

Fig. 2 Picture a) will by probably more appropriate in same magnification as b)

Can you add a small space between b) and c) to differentiate the two Sts?

Response: The picture (a) has been replaced with another one whose magnification is the same as the other two pictures in Fig. 2. The spacing between b) and c) has also been corrected. The figure is enclosed herewith.

The delta precipitates

I think the EDS results shall be given as well, if you are using them to distinguish the Laves phase and delta phase.

Response: A typical example of the EDS result is included now as Fig. 5 and quantitative data is presented in Table 2 in the manuscript.

Line 269 – “larger”, not “lager”

Response: corrected!

Figure 3

Can you give the orientations of the depicted specimens?

Response: The orientation of this image (for example relative to the building direction) cannot be provided now as it is hard to trace back. In addition, we didn’t bother about the orientation for this purpose. The microstructure in Fig. 3b actually shows columnar subgrains, not grains. Due to low misorientation (<5 degrees) between adjacent subgrains, it is difficult to resolve the crystallographic orientations of subgrains with EBSD.    

Please add arrows to Laves phase particles in a) and c), and for GB in d)

Response: We provided another similar image from which we have performed EDS analysis.

Details will be useful to support the differences in delta precipitates, or numerical values.

Response: EDS data is added to the manuscript now. See Fig 5 and Table 2.

Can you add any supporting evidence for the sentence on line 285 and 286?

Response: As we have stated, the estimation was simply from visual estimation, and we didn’t carryout quantitative analysis to support the sentence. We believe that quantitative analysis is not easy to do.

Any comment to preferential orientation of delta precipitates?, Fig. 3b

Response: This is considered and commented in the manuscript (right after Table 2).

Any comment to figure 4? Why is it given here? Is it from a broken tensile specimen?

Response: Fig. 4 has been relocated to section 3.5 (Fracture analysis) and renamed as Fig. 14.

Fig. 5 – Where is the Laves phase reported in Line 291 to 298.

Response: We have previously reported (in three articles) on the microstructure and phases of as-printed In718. For clarity, reference #21 is cited, which shows subgrains enriched in the laves phase.

Fig. 5 – Orientations of the specimens 5a and 5b?

Response: Please note that the images were taken with TEM. Secondly, the precipitates shown are along subgrain boundaries, whose preferred orientations cannot be determined with TEM or EBSD. Note also, the image in 5a shows cellular subgrains, whereas 5b shows columnar subgrains. If the comment is about precipitates, some of them are in line with subgrain boundaries, and others seem aligned at an angle of approximately 45 degrees. We didn't go into detail on the nucleation and growth (coarsening) of the delta precipitates since the topic was rather aimed at reporting the effects on the mechanical properties, particularly on impact toughness.

N

Line 294 – “well known..” – any reference?

 Response: References #8, 23 and 31) are sited now.

Line 299 to 307 – belongs rather to Discussion section, but can be here.

Response: Agree to keep where it was!

Line 308 – 314 – Maybe, will be better in the beginning of the 3.1 section, or at the beginning of the Delta phase section

Response: We believe that this part of the text fits well with the current position rather than the suggested position. It is a comparative argument that shows the effects of solid solution heat treatment at two different heat treatment temperatures. Therefore, we prefer to keep it in the current position.

The annealing twins section – will be probably more appropriate between the Microstructure section and Delta phase section.

Response: Agree and it has been relocated. Fig. 6 becomes Fig 3, after relocating.

Page 10 of 2022

Line 336 and 338 - Probably “solution” instead of “solidification”

Response: Yes, it should be solution, not solidification. Correction is made now.

Line 338-339 - Any evidence or reference to “However, the subgrain boundaries…”.

Response:  We have edited that sentence as follows:

However, the subgrain boundaries have coarsened, but the dislocation networks entangled at the subgrain boundaries appeared partially annihilated. Some of the large and irregularly shaped precipitates shown along the subgrain boundaries may also be undissolved Laves phase.

Coarsening of the subgrains is evident from the TEM image in Fig. 5, based on the size. The average size of the as-printed subgrains of L-PBF-fabricated IN718 is about 0.5 µm. After the ST at 980 degrees C, the size of the subgrains is about 1 µm or larger as shown in the images. The subgrain boundaries shown are very clear, which means they are exhibited without dislocation or with minimal dislocation networks compared to that of the as-printed material (reference #8, for example).

Identification of delta phase

Please add a space between the title and previous paragraph.

Response: It is corrected now.

This part is also about identification of other phases, as gamma double prime

 

Phase identification with X-ray

Maybe, will be more useful before the Delta precipitates section, since this X-ray measurement characterises bigger area and shows, that there is no delta in SS1100 specimen.

Response: It sounds correct and we moved the X-ray measurement before the electron diffraction section.

The texture coefficient – Maybe, will be more illustrative from EBSD? Since EBSD was provided in Twins section.

Response: Discussing the details of crystallography is outside the scope of this article and we prefer to keep the existing content.

Line 382 – I do not see the “Obtained numerical ?results?”

 Response: The calculated values of R(hkl), TC(hkl) and TCR(hkl) for both specimens are given in Table 3.

Line 399 to 409 – Is rather Discussion, discusses delta, grain size, gamma ‘’.

Line 419 – 428 – Is rather Discussion.

Line 460 – 471 – Is rather Discussion.

Fig. 11 – Will be more useful next or closer  to Fig 10.

Line 487 – 494 - Is rather Discussion.

Response: It is true that the paragraphs mentioned could be in the ‘Discussion’ section. We provided such descriptions under the ‘Results’ section to highlight the results in general and intend to emphasize ‘annealing twins’ and ‘delta phase’ in the ‘Discussion’ section to stress the objectives of the study. So, we prefer to keep it as it stands now.

3.5 Fracture analysis

Fig. 4 – Will be more useful in this section

Response: Fig. 4 has been moved to the ‘Fracture analysis’ section and renamed as Fig. 14.  

Line 508 - Any evidence from the fracture surface about the IG character of the microcracks and presence of larger delta precipitate?

Response: We have shown the existence of larger delta precipitates along the grain boundaries in the previous section (e.g., Fig. 4a). The role of delta phase is also discussed. However, additional contribution may also come from the undissolved laves phase. Realizing this, we have added the following sentence to the second paragraph of Section 1.5.

It should also be noted that some of the undissolved phases along the grain boundaries do contribute to the microcracking observed along the grain boundaries. The Laves phase is known to be detrimental to the mechanical properties of Inconel 718.

Fig. 13 - More detailed images are needed for documentation of the “area of cleavage”. Any role of carbides?

Response: All the images in Fig. 13 have been replaced with newly acquired SEM images of equal magnification and better resolution than the old ones. The cleavage facets are now clearly visible. An inset image is included in Fig. (c) to illustrate one of the facets in (c). Fig. 13 is also enclosed herewith.

The effects of carbides are not considered in this work, assuming a low level of concentration compared to other precipitates. The effects between the two sets of specimens (S980, S1100) may also not be very significant, as carbide is stable at the corresponding temperatures of solid solution treatment we have performed, as indicated in the literature. This means that the effects of the carbide phase on both sets of specimens are nearly the same.

Line 522 – 531 - Is rather Discussion

Response: It is true that the paragraphs mentioned could be in the ‘Discussion’ section. We provided such descriptions under the ‘Results’ section to highlight the results in general and intend to emphasize ‘annealing twins’ and ‘delta phase’ in the ‘Discussion’ section to stress the objectives of the study. So, we prefer to keep it as it stands now.

Fig 14, Line 544 – Typing error “coalesces”, or any word missing?

Response: It is now corrected as ‘cleavage facets.

Fig. 14 – More detailed images are needed for documentation of the “?facets?”. Any role of carbides?

Response: The sporadically distributed cleavages are clearly exhibited in the images and are indicated using arrows. It could be beneficial to present better images, but we do not have a better image to improve clarity beyond what is shown in the figures.

4.1

Line 563 – 566 – This is evidence of IG character of secondary microcracks. As a consequence of the specimen orientation?

But on fracture surface, mostly ductile dimples were shown. Probably TG?

Response: Both types of fractures could be possible. In the given image showing the enrichment of delta precipitates along the grain boundaries, the fracture could be intergranular. On the other hand, since S980 contains small grains and larger grains, which allows strain localization, it may initiate and propagate transgranular fracture. Transformation from transgranular to intergranular fracture may also occur. We have included this possibility in the manuscript!

We are unsure about the comment regarding the specimen orientation in relation to the fracture!

Line 570 – What is the base for “attribution of high ductility to presence of twins”? References? Evidences? In previous parts of the Results section you have discussed also grain size. How did you separate this effect? Not talking about the gamma double prime. I think this separation is insufficient.

Response: Regarding the attribution of twins to the high ductility, we have provided sufficient evidence by citing several works (references #24, 45-48, 49-51). The discussions between lines 589 and 613 clearly address this topic and present evidence from the literature. The relevant literature has been cited and discussed. If the reviewer knows of other references, we ask them to let us know so that we can include them in the paper.

Grain size: The effects of grain size were discussed by comparing S980 with S1100. Here, we have stated factors contributing to the highest mechanical strength obtained with S980. In our previous studies (references 8 & 9), we have reported the effects of solution heat treatment temperature and/or hold time on hardness, in which coarsening of grains is viewed as one of the factors. However, separating the contributions of different factors is difficult based on the current heat treatment design.

Possible influence of other factors is mentioned in the Conclusions part as well as in Abstract. Please, make all parts more consistent to the other parts.

 Response: We have addressed the factors in the other comments. We prefer to emphasize the effects of delta and twins in the abstract and have highlighted them in the conclusion. Both sections have been revised.

Conclusions

How do you conclude which factor is “mainly responsible”?

Response: The Conclusion section has been revised to correct the phrase "mainly responsible."’

Grain size, subgrains…It seems contradictory to the Title of the article and the introductory paragraph of the 5.Conclusions section.

Response: It is generally not possible to assess the effects of the delta phase alone without considering the effects of other factors. Therefore, it is necessary to consider other factors in addition to delta. However, we have revised the Conclusion section to focus more on the title's objective.

Last point – Is true, but some advice that no delta alloy is not the best choice is e.g. in Ref 20.

Response: We have revised that part now. Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you, authors for the improved article. A lot of work has been done. I still have found a few minor issues.

Line 73 – gamma prime is usually referred as FCC, not primitive cubic (Donachie, pp34)

Line 146 – excessive dot after parentheses Tabel 1 – Can be wide to the page width

Line 184 – type of notch should be referred also here, U or V.

Line 205-211 – Ageing treatment – I believe you can find also other references to discuss this issue and involve it in the Discussion section.

Line 371 – Title numbering is not connected to previous sections from this point to the end of the article Line 374 – I do not see MC carbides in Fig. 7

Line 434 to 442 – Nice piece of Discussion, will be useful in Discussion section to discuss also other mechanical properties.

Line 480 to 490 – the same comment as above

Line 566 – “High magnification images (not shown)… - please show them

Line 603 – The grains ….exhibited significant growth…. – Here the discussion of grains can easily continue.

Line 615 to 617 – The sentence is not fair, Li has used EOSint -M280 SLM machine, you have used EOS M290 machine. Therefore, it is quite easy to compare the grain size. Moreover, Li in referred article [24] attributed the good plasticity to both „recystallized grains with annealing twins“ in Conclusions. If you want to separate the twins’ effect from grains effect, maybe you should provide some more quantitative information about the sigma 3 grain boundaries distribution fraction, like Fig. 6 in [24].

Line 658 – unreleased stress – Can there be any unreleased stress after 980C annealing?

Line 674 – 676 – The same is valid for the twins  

Some general comments

As you experienced recrystallisation during SS1100 some part of the Introduction should be dedicated to this phenomenon and its connection to mechanical properties. Whether it is excluded, or not.  

In my opinion, Discussion section is to discuss all possible influences (grain size, twins, delta phase, gamma prime and double prime). Whether are some (if any) more important than others shall be obvious at the end of the Discussion section, not at the beginning or even before beginning.

Obviously, twins have an effect, but grains too. I do not understand why are you omitting them from frank and proper discussion.

 

Author Response

Reviewer 3 (roun2)

We are once again very thankful to the reviewer for the thorough reading and for providing us with more comments and feedback that improve the quality of the paper. The comments have been addressed, and all necessary corrections have been made in the new version. The sections of the paper that have been revised in the manuscript are highlighted in grey. Our responses to each of the comments are in boldface below.

Line 73 – gamma prime is usually referred as FCC, not primitive cubic (Donachie, pp34)

Ans: corrected accordingly!

Line 146 – excessive dot after parentheses Tabel 1 – Can be wide to the page width

Ans: Table 1 has now been expanded to full-page dimensions.

Line 184 – type of notch should be referred also here, U or V.

Ans: It is added now, and rewritten as ‘For the V-notch impact toughness tests …’

Line 205-211 – Ageing treatment – I believe you can find also other references to discuss this issue and involve it in the Discussion section.

Ans: We intentionally omit discussion of gamma precipitates in detail, such as size and volume fraction quantification, due to differences in aging temperature and holding time. We want to emphasize the main differences in mechanical properties, specifically ductility and impact toughness, which may be due to differences in delta precipitates and microstructure (twins) as we have stated elsewhere in the paper. 

Line 371 – Title numbering is not connected to previous sections from this point to the end of the article Line 374 – I do not see MC carbides in Fig. 7

Ans: The numbering is corrected now.

MC carbides: the volume fraction of carbides is so low in this sample and the corresponding peak intensity is quite weak, but it is believed to be overlapped with the delta peak. This was shown as δ+NbC, unfortunately, 'C' was placed at the back of a nearby peak. Fig. 7 has been edited and it is visible now.

Line 434 to 442 – Nice piece of Discussion, will be useful in Discussion section to discuss also other mechanical properties.

Ans: Based on the reviewer’s suggestion in R1, we found it reasonable to move the contents from line 434 to 442 to the Discussion section under 4.2. See the first 3 paragraphs.

Line 480 to 490 – the same comment as above

Ans:  Similar to the above response, we found it reasonable to move the contents from line 480 to 490 to the Discussion section under 4.2 as the first paragraph. See the first 3 paragraphs.

Line 566 – “High magnification images (not shown)… - please show them

Ans: An example of a high magnification image is added as an inset in Fig. 15 (e). Line 566 is also edited accordingly. 

Line 603 – The grains ….exhibited significant growth…. – Here the discussion of grains can easily continue.

Ans: This has been done following comments provided above on the content from lines 480 to 490.

Line 615 to 617 – The sentence is not fair, Li has used EOSint -M280 SLM machine, you have used EOS M290 machine. Therefore, it is quite easy to compare the grain size. Moreover, Li in referred article [24] attributed the good plasticity to both „recystallized grains with annealing twins“ in Conclusions. If you want to separate the twins’ effect from grains effect, maybe you should provide some more quantitative information about the sigma 3 grain boundaries distribution fraction, like Fig. 6 in [24].

Ans: Grain size - We are still insisting on not comparing the grain sizes from the two studies. Primarily, due to the significant differences in ST temperatures (1100°C vs. 1150°C), which can result in different levels of recrystallization and grain growth. Secondly, the printing conditions, especially process parameters, could be different even though the AM machines could be similar in model.

Ans: Quantification of annealing twins - Images of S980 and S1100 illustrating the presence of annealing twins in Fig. 3 are believed to be sufficient to show the significant differences in microstructure between the two specimens in terms of twinning. However, to address our reviewer's comment, we have quantified the twins in the two specimens and added as Fig. 3(c). The two images were also reprocessed by OIM to generate CSL Σ3, 5, 7, 9, and 11. Red lines indicate Σ3 twin boundaries, and green lines indicate the twin boundaries of the rest of the CSL sigma.

Figure caption of Fig. 3, is revised as follows:

Fig. 3: EBSD quality images (1.2 x 1.2 mm) of specimens oriented parallel to the BD (a) ST at 980°C/1h and (b) ST at 1100°C/3h (c) Number fractions of various sigma (S) values. In (a, b) general grain boundaries are marked by black lines, S3 twin boundaries by red lines and the rest of S twins by green lines. The arrow in (a) shows the building direction during printing. The inset in (c) is a magnified plot showing twins with a low number fraction. 

The following paragraph is added as the last paragraph of Section 3.1.2 to describe Fig. 3(c).

Comparison of different CSL values for the two specimens is presented in Fig. 3(b). As shown in Fig. 3(a), S980 is dominated by general grain boundaries (black lines), resulting in a very small quantity of CSL twins observed (<2%). In contrast, the number fraction of CSL in S1100 is significantly higher compared to S980. Specifically, the CSL value of Σ3 (red lines) is 69.3% in S1100 compared to 1.34% in S980. As we will see later, the variations in the quantity of twins can demonstrate different impacts on the mechanical properties.

In connection with the above, the following paragraph is added to the Discussion section (4.2)

The grains of the specimens ST at 1100°C exhibited significant growth, accompanied by a high density of annealing twins, as shown in Figures 2 and 3. This contrasts with the specimens ST at 980°C as shown in Fig. 3(c). The annealing twins induced in S980 are generally none or very insignificant (~1%). On the other hand, S1100 exhibits a large volume fraction (>69%) of S3 annealing twins. For IN718, as we have shown previously [Tucho, 2019 #7], the solution treatment temperature of S980 is not high enough to provide enough thermal energy for significant recrystallization. The solution treatment temperature of 1100°C for a holding time of 3 hours however supplies enough thermal energy to ensure a near completion of recrystallization of S1100 specimens. This temperature scheme is believed to be optimal for L-PBF-based IN718 for reengineering grain boundaries to obtain the maximal density of annealing twins.

Line 658 – unreleased stress – Can there be any unreleased stress after 980C annealing?

Ans: Yes, S980 is assumed to be under more stress than S1100 due to different levels of recrystallization. In relative terms, the stress in S980 is higher than that of S1100 after the solid solution heat treatment at different temperatures. Because of the dissolution of the Laves phase, S1100 is nearly strain-free in this regard. However, S980 is not, as the Laves phase was only partially dissolved. Due to the lattice mismatch between the matrix and the Laves phase, S980 still experiences high-stress conditions. Most of the lattice defects, especially subgrain boundaries, are still present in S980, but not in S1100.

Line 674 – 676 – The same is valid for the twins  

Ans: Under normal circumstances, annealing twins are free of strains/stress, but they can embed dislocation motion and can be site for dislocation storage.

Some general comments

As you experienced recrystallisation during SS1100 some part of the Introduction should be dedicated to this phenomenon and its connection to mechanical properties. Whether it is excluded, or not.  

In my opinion, Discussion section is to discuss all possible influences (grain size, twins, delta phase, gamma prime and double prime). Whether are some (if any) more important than others shall be obvious at the end of the Discussion section, not at the beginning or even before beginning.

Obviously, twins have an effect, but grains too. I do not understand why are you omitting them from frank and proper discussion.

Ans: As we have shown above, factors other than delta precipitates and twins, which were raised in the ‘Results’ section, have now been moved to the 'Discussion' section to address the reviewer's comments.

Author Response File: Author Response.pdf